1. Field of the Invention
The invention relates to nanowires, and more particularly to controllable artificial dielectrics using nanowires.
2. Background Art
Dielectrics are materials that are used primarily to isolate components electrically from each other or to act as capacitive elements in devices, circuits and systems. A unique characteristic of a dielectric is that nearly all or a portion of the energy required for its charging from an external electrical field can be recovered when the field is removed. Dielectrics have an extremely wide range of applications, including but not limited to, electrical components used in communications to radar absorbing materials (RAM).
Example dielectric materials include polyethylene, polypropylene, polystyrene, cross-linked polystyrene, fused silica, fused quartz, Alumina (Al2O3), Boron Nitride (BN), Beryllium Oxide (BeO) and Magnesium Oxide (MgO). Polyethylene is one of the most common solid dielectrics, which is extensively used as a solid dielectric extruded insulant in power and communication cables. Polypropylene also has many electrical applications both in bulk form and in molded and extruded insulations as well as in film form in taped capacitor, transformer and cable insulations. Alumina is used for dielectric substrates in microcircuit applications. Magnesium oxide is a common inorganic insulating material which is utilized for insulating heating elements in ovens. A wide range of dielectric materials exist with a wide range of applications. Further discussion of dielectric materials and their uses can be found in R. Bartnikas, Dielectrics and Insulators, in The Electrical Engineering Handbook 1143-1150 (Michael C. Dorf, ed. CRC Press 1993), which is herein incorporated in its entirety by reference.
Two fundamental parameters characterize a dielectric material. These are conductivity, σ, and the value of the real permittivity or dielectric constant, ∈r. The conductivity is equal to the ratio of the dielectric's leakage current density J1 to an applied electric field E. Equation (1) provides this relationship:σ=J1/E  (1)
The dielectric constant, ∈r, is determined from the ratio:∈r=C/Co  (2)
where, C represents the measured capacitance in fahrad (F) and Co is the equivalent capacitance in vacuum, which is calculated for the same specimen geometry from Co=∈oA/d. ∈o denotes the permittivity of the dielectric in vacuum and is equal to 8.854×10−14 Fcm−1. A is the surface area of the dielectric and d is the thickness of the dielectric. Liquid and solid dielectric materials typically have dielectric constants ranging from approximately 2 to 10. Example approximate dielectric constants for common dielectrics at 20° C. with a 1 Mhz signal are as follows.
DielectricDielectric ConstantAlumina8.5Magnesium Oxide9.69Polypropylene2.3Polyethylene2.25
An important dielectric characteristic is the dielectric loss or dissipation factor. Under alternating current (AC) conditions dielectric loss arises mainly from the movement of free charge carriers (electrons and ions), space charge polarization, and dipole orientation. Ionic, space charge, and dipole losses are temperature and frequency dependent, a dependency which is reflected in the measured valued of σ and ∈r. This necessitates the introduction of a complex permittivity defined by∈=∈r−j∈i  (3)
where ∈i is the imaginary value of the permittivity.
The complex permittivity, ∈, is equal to the ratio of the dielectric displacement vector D to the electric field vector E, as given by∈=D/E  (4)
Under AC conditions the appearance of a loss or leakage current is manifest as a phase angle difference between the D and E vectors. D and E may be expressed as:D=Doexp[j(ωt−δ)]  (5)E=Eoexp[jωt]  (6)
Where ω is the radial frequency term, t the time, δ is a phase difference between D and E, and D0 and E0 the respective magnitudes of the two vectors. From these relationships it follows that∈r=Do/Eo*cos δ  (7)∈i=D0/E0*sin δ  (8)
Under AC conditions the magnitude of loss of a given material is defined as its dissipation factor, tan δ From equations (7) and (8), the dissipation factor can be represented astan δ=∈i/∈r=σ/ω∈r  (9)
These basic characteristics and example dielectric materials illustrate the wide range of dielectrics, applications and characteristics. Nonetheless, the desire to employ dielectrics with more robust operating characteristics for evermore increasing applications combined with the physical limitations of dielectric materials, led to the development of a wide variety of dielectric structures and the creation of artificial dielectrics.
An example of a dielectric structure developed for shielding electromagnetic energy is provided in U.S. Pat. No. 5,583,318, entitled Multi-Layer Shield for Absorption of Electromagnetic Energy, issued to Powell on Dec. 10, 1996 ('318 Patent). The '318 patent teaches a multi-layer structure for shielding electromagnetic energy in a data processing equipment enclosure. The multi-layer structure is designed to substantially reduce spurious transmissions from a source within the data processing equipment enclosure by absorbing the electromagnetic energy and dissipating the electromagnetic energy as heat within the multi-layer structure. The multi-layer structure is formed by stacking various layers of dielectric, conductive and/or polymer materials together. The effective impedance of the multi-layer structure is changed by selecting different combinations of materials to change the effective properties of the shielding structure.
Another example of a dielectric structure and process for producing such a structure is given in U.S. Pat. No. 6,589,629, entitled Process for Fabricating Patterned, Functionalized Particles and Article Formed from Particles, issued to Bao et al., on Jul. 8, 2003 ('629 Patent). The '629 patent teaches a technique for forming functionalized particles, where such particles are readily formed into periodic structures. The functionalized particles are capable of being formed into an ordered structure, by selection of appropriate complementary functionalizing agents on a substrate and/or on other particles. This process can be used to develop photonic band gap (PBG) materials. PBG materials are potentially useful as waveguides and microcavities for lasers, filters, polarizers, and planar antenna substrates. A PBG material is generally formed by combining a high refractive index dielectric material with a three dimensional lattice of another material having a low refractive index, to form a three-dimensional Bragg grating. The propagation of light in the PBG structure therefore depends on the particular energy of the photon.
Investigations have been made into the formation of artificial dielectric composites by the inclusion of high-aspect ratio conductive filaments into polymer matrices. W. Stockton, et al., Artificial Dielectric Properties of Microscopic Metalized Filaments in Composites, 70 (9) J. Appl. Phys. 4679 (1991). Stockton examined two size ranges of filaments, commercially available fibers with diameters near 10 μm, and self assembling microstructures, derived from organic surfactants, whose diameters were about twentyfold smaller. Stockton at 4679. Stockton noted that both high and low dielectric loss composites are possible depending on filament conductivity. Id. Stockton further noted that in a complex material such as the fiber/polymer there is a large number of experimental values—particle diameters, length distributions, aspect ratios, metallization thickness, conductivity, particle dispersion, clustering, alignment, polymer matrix dielectric properties, and density—that affect composite permittivities. Id. Stockton used metal films (e.g., nickel, copper, iron or permalloy) to create metalized tubules or fibers for use as the filaments to be embedded within the dielectric material. Id. at 4681.
Similarly, the effect of adding metalized tubules to an insulating polymer were examined by Browning et al. S. L. Browning et al., Fabrication and Radio Frequency Characterization of High Dielectric Loss Tubule-Based Composites near Percolation, 84 (11) J. Appl. Phys. 6109 (1998). Browning observed that when sufficient particles have been loaded the composite of tubules and polymers will begin to conduct over macroscopic distances. The onset of this transition is called percolation, and the volume loading of conducting particles at this point is termed the percolation threshold. Browning noted that percolation is accompanied by substantial changes in dielectric properties. Browning at 6109. Browning examined permittivities as a function of loading volume of the tubules. Id. at 6111.
Existing artificial dielectrics are limited in their application and use because they typically have relatively low dielectric constants and have fixed dielectric constants at a particular frequency. These limitations, as discussed in Stockton, have been attributed to clumping of metallized fibers once the density of the fibers becomes high. When the metallized fibers clump, they lose their insulative characteristics and become conducting, thereby limiting the ability to achieve high dielectric constants within existing artificial dielectrics. Furthermore, dielectric structures based on existing dielectrics and dielectric structures are limited in their application.
The '432 Application addressed the need for artificial dielectrics to have very high dielectric constants that can be statically or dynamically adjusted based on a wide range of potential applications. An additional need exists to provide continuously variable permittivities within an artificial dielectric.
What are needed are artificial dielectrics that have spatially varying permittivities.
What are also need are dielectric materials that are advantageous over conventional dielectric materials with regard to cost, ease of processing and mechanical properties.